To measure the GFR utilising the clearance principle, the substance utilised should have the following characteristics:

A substance that satisfies these conditions is Inulin, a polymer of fructose. By injecting an amount of inulin and then measuring the arterial concentration of inulin (after a steady state has been reached) and measuring the exact volume of urine passed during a period of time and measuring the concentration of inulin in urine, the GFR can be calculated.

Another substance that can be utilised is creatinine. Creatinine is a metabolite of muscle metabolism, is freely filtered at the glomerulus and only slightly secreted at the tubule, its clearance is closely matched to the GFR, it is clinically utilised to measure GFR as it is more convenient than inulin.